# Asymmetry in p-p, n-n and n-p interactions

Why does the asymmetry exist between the proton-proton, neutron-neutron and neutron-proton strong interactions in the nucleus (which results in nuclei preferring equal numbers of neutrons and protons)? Why is the neutron-proton interaction the strongest?

• which results in nuclei preferring equal numbers of neutrons and protons No, this is not the reason why light nuclei have $N\approx Z$. See physics.stackexchange.com/questions/323350/… . – Ben Crowell Jul 10 at 17:36
• Why is the neutron-proton interaction the strongest? This is not really true in general, or even a well-defined statement. We do see states like the deuteron in which there is a strong attraction between the proton and neutron, but we can't make an apples-to-apples comparison with, e.g., a pair of neutrons in the same state, because that would be forbidden by the exclusion principle. – Ben Crowell Jul 10 at 17:39
• york.ac.uk/physics/research/nuclear/proton-neutron-symmetries "For example, it is well known that the nuclear force, at least when measured in free space, is not quite charge symmetric - that is, the neutron-neutron interaction is just a bit stronger than the proton-proton interaction." – user50229 Jul 11 at 19:45
• @BenCrowell en.wikipedia.org/wiki/… "The symmetry correction term -d(N-Z)^{2}/A^{2} takes into account the fact that in the absence of other effects the most stable arrangement has equal numbers of protons and neutrons; this is because the n-p interaction in a nucleus is stronger than either the n-n or p-p interaction." – user50229 Jul 11 at 19:56